For transmission of video (audio and video) data on the Internet, a download type transmission method and a stream type transmission method are currently employed.
In the download type transmission method, a video file transmitted from a distribution server is once copied at the terminal and, thereafter, data of the video file (video data) is reproduced. So, the terminal cannot perform data reproduction until the file transmission is completed. Therefore, the download type transmission method is not suitable for long-hours of reproduction of video data or the like.
On the other hand, in the stream type transmission method, while video data or the like is transmitted from a distribution server to a terminal, received data is reproduced at the terminal.
Recently, a stream type transmission method using, a protocol called RTP (Real-time Transport Protocol) which is defined in IETE-RFC 1889, has mainly been used.
FIG. 28(a) is a diagram for explaining a video data transmission method according to the RTP.
With reference to FIG. 28(a), a distribution server (transmitting end) and a terminal (receiving end) such as a personal computer are connected by a cable in the Internet, such as modem, ISDN, or LAN, and transmission of video data is carried out between the server and the terminal by using the RTP.
In the data transmission according to the RTP, processes for the respective packets are synchronized between the transmitting end and the receiving end by using time stamps as time information, and asynchronous (late arrival) packets and packets affected by transmission errors are discarded at the receiving end. Further, discarded or lost packets are detected at the receiving end, according to the absence of sequence numbers give to these packets.
On the other hand, Internet applications utilizing mobile phones, for example, mail access and text information service, are advancing now, and infrastructure for high-speed radio data communication (high-speed packet communication of about 384 kbps) is under preparation for practical use of the third generation mobile communication (W-CDMA: Wide band-Code Division Multiple Access).
FIG. 28(b) is a diagram illustrating a communication network for the above-described W-CDMA visual terminal.
Such communication network includes a radio transmission section. For example, when performing data transmission between a video distribution server and a visual terminal through a relay server, the section between the distribution server and the relay server is a cable transmission section by the Internet, but the section between the relay server and the visual terminal is a radio transmission section by a mobile phone network such as the W-CDMA.
However, the bit error rate in the radio transmission section is 10−3 while the bit error rate in the cable transmission section is 10−5–10−7, and the radio transmission quality becomes a problem in the RTP type data transmission method in which the reproduction quality depends on the end-to-end (server-to-terminal) transmission quality.